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Anti-Electrons Help Researchers Find Nano-Surface Defects in Gold

OAK RIDGE, Tenn., Dec. 21, 1999 — Tiny defects in the surface of common material - from silicon to steel - determine the properties of material and how it can be used. Unfortunately, many of these pores, called vacancies, are so small they cannot be accurately measured.

In the past, measurement of vacancies has seriously limited the development of new or improved materials, such as the next generation of optical and electronic devices. This has been especially significant when the vacancies are in devices that are only a few nanometers (one millionth of a millimeter) in size.

Today an international team of researchers at the Department of Energy's Oak Ridge National Laboratory (ORNL) is applying the use of positrons, or anti-electrons, to this task with considerable success.

In a paper published in the November 29th issue of Physical Review Letters, a team from ORNL; Lucent Technologies, Inc.; Fisk University; and Japan's Electrotechnical Laboratory document an experiment using positrons to find clusters of four atomic vacancies at the surface of gold nanoparticles embedded in a magnesia matrix. These clusters of vacancies explain changes in the optical properties when the materials are subjected to different fabrication processes.

Positrons were generated by smashing gamma rays against a tungsten target at the laboratory's Oak Ridge Electron Linear Accelerator. The gamma rays divide into negatively charged electrons and their anti-matter, positrons. The decay of unstable sodium 22 provided an alternative source of positrons. The positrons are injected into the gold nano-particles, and through advanced spectroscopy, the researchers are able to determine the size, location, and concentration of the vacancy clusters.

Nanoscale science and technology is an area of interest in the Office of Science at the Department of Energy, which supported the project. Possible future applications for this work include higher speed computer chips, manipulating properties of optical devices, less brittle ceramic material, and improved fiber composite materials.

The work was done under the leadership of physicist Dr. Jun Xu of ORNL's Chemical and Analytical Sciences Division.

Oak Ridge National Laboratory is a multiprogram research facility of the Department of Energy managed by Lockheed Martin Energy Research Corporation.